Method of making a grid structure



NOV- 25, 1952 s. F. VARIAN ETAL 2,619,438

METHOD OF MAKING A GRID .STRUCTURE Filed April 16, 1945 fav INVENTORS/GURD E QR/,4N

l ATTORNEY Patented Nov. 25, 1952 METHOD OF MAKING A GRID STRUCTURESigurd F. Varian and Russell H. Varian, Garden City, N. Y., assignors toThe Sperry Corporation, a corporation of Delaware Application April 16,1945, Serial No. 588,580

1 Claim. 1

The present invention relates to control electrodes for electron tubesand to methods for producing the same. Cellular grid structures of thetype here disclosed are especially adapted for use as the controlelectrodes in thermionic tubes, such as klystrons, but also have manyother uses.

In the past, it has been common practice to make the control electrodesor the grids of electron tubes either from fine Wire screening or byfabrication. In general, such structures have been relativelyunsatisfactory for the purpose. Both types of construction lack suicientrigidity when the supporting Wires or interstitial metal or webs are soreduced in size and volume as to permit the development of maximumdesired interstices or pores. Neither of these two types of constructionprovides a grid which has at least one regular geometrical surface. Forexample, a perforated sheet normally cannot have a regular geometricalsurface, since the surface is deformed by the punching operation. Anattempt to remove the deformation of the sheet which results from theunilateral burring due to the perforating operation almost invariablycauses irregularities in the size and shape of the perforations. When awoven Wire screen is used, the weaving operation bends the wires andthus inherently eliminates the possibility of developing a regulargeometrical surface on either side of the screen. This is also true ofother types of wire screens. The shape of the openings has also beenfound to be of importance. Experiments have shown that bettercollimation of the electron beam is achieved when hexagonal openingsthan with the previously more readily obtained circular or rectangularconfigurations.

`Grrids and other control electrodes previously used have beencharacterized by a relatively high electron interception factor. Toolarge a portion of the total area occupied by the structure wasnecessarily utilized to provide proper structural stability and othermechanical properties, so that the area available for the passage ofelectrons was unduly restricted. As electron density and velocityincrease with improvements in electron tubes, it becomes increasinglyimportant to minimize the interception factor of the grid and othercontrol electrodes. At the same time, the necessity for providing astructure which has at least one regular geometrical surface and highstructural rigidity is increased, since dimensional tolerances and thetendency toward microphonic effects are increasingly important problemsunder these conditions.

Accordingly, it is a feature of the present inventon to provide animproved grid member or foraminous disc of multicellular constructionwhich has desirable mechanical properties resulting in reducedmicrophonic effects and a relatively low interception factor.

It is another feature of the present invention to provide a method ofproducing such grids or foraminous discs on a quantity basis with eX-tremely good uniformity.

The invention contemplates the formation of cellular grid members,desirably in disc form, and in the following manner. A plurality ofwires of a first metal are employed as mandrels which are later removed.Each of the wires is plated or otherwise coated or sheathed with asecond metal. In a, preferred form of the invention, the coated wiresare bunched and surrounded by a suitable sheath or housing. Theresultant assembly is then consolidated, as for example by drawing orswaging, after which wafers of a suitable thickness are cut off. Thewafers are then subjected to a chemical `action Which preferentiallydissolves or etches the Wires out of their coatings, leaving a cellular,honeycomb-type 1 structure formed from the original coatings or sheathsof the Wires. The adjacent Walls of the coatings, in turn, areautogenously joined or coalesced, as for example by subjecting thewafers to heat treatment such as sintering. If desired, the coalescingmay be accomplished before the individual wafers are sliced off, orafter slicing but before etching is accomplished.

The foraminous disc or grid member in accordance with the presentinvention is formed by a, plurality of tubular or tubiform sections orcells having substantially parallel longitudinal axes and with theiradjacent wall surfaces joined together. The sections have relativelythin Walls, so the interception factor` of the grid member is desirablylow. Since the axial length of the sections is preferably of the orderof their opening size, however, the grid member has substantialstructural rigidity, each section being supported by, and in turncontributing to the support of, several adjacent sections.

The above features and brief description of the invention will be betterunderstood by reference to the following detailed description taken inconnection with the accompanying drawing, in which like components aredesignated by like reference numerals and in which:

Fig. l is a flow chart, in diagrammatic form, of the method of thepresent invention;

Figs. 2, 3 and 6 are partial sections taken respectively on lines 2 2, 33 and -6 of Fig. 1;

Fig. 4 is a section of a curvilinear grid structure made in accordancewith a modification of the method of Fig. 1;

Fig. 5 is a section of a curvilinear grid structure made in accordanceWith another modication of the method of Fig. 1;

Fig. 'I is a section taken on line 'I-'I of Fig. 6; and

Fig. 8 is a plan view of a foraminous disc. or grid structure inaccordance with the present invention.

Referring now to Fig. 1, the first step in the method of producingforaminousv discs is the selection of a plurality of wires I'. Thesewires are of a soft malleable materiaLsuch as zinc or aluminum, althoughiron may also be employed. The wires preferably have a` substantiallycircular cross section, but other shapes may be employed if desired. Itis not absolutely essential that wires Ill be metallic. Wire-like rodsor filaments of other materials, asv for example fibers of. hemp.cotton, etc., may be utilized with satisfactory results.

The second step of the method comprises coating wires I0 with anothersuitable material, this step being indicatedgenerally by unit I-I. Thecoating may be accomplished in a number of Ways,jincludingelectroplating, dipping in molten metal, anodizing, metalizing, orpainting with a fluid mixture ofY powder and binder.v In some cases, thecoating may be applied to wires I by Winding avribbon of the coatingmaterial spirally around' each wire and spot welding its ends to hbld itinplac'e. Thecoating material may either be al good conductor, such ascopper, gold orl silver; 0r metal having a high melting point but beingof onlyl fair conductivity may be employed, such for example as nickel,platinum, molybdenum, tungsten or 'rhenium. The coating material neednot necessarily be metallic. Graphite grids, for' instance, may beproduced by vcoating the Wires with colloidal graphite, such for exampleas AquadagL Aquadag is a dispersion of water-graphite paste in water,and it comprises ai solution of substantially 22% graphite in water. Theessential requirements are that the coating be substantiallyhomogeneousv and that it be malleable and ductile. The thickness of thecoating is normally made approximately half the desired grid webthickness. The coated wires are next passed through an assemblyingfunnel I2 'and into 1a sheath I3. The 'sheath I3 comprises va tube ofsome material which can be read-ily drawn, as for example copper. Thecoated Wires need not necessarily complet'ely ll sheath I3. Theappearance of the bunch'ed and sheathed wires at this point is indicatedin Fig. 2, in which the coatingv on the Wires is designated by thereference numeral I4.

The sheath I3` and its contents are now consolidated, as for example bydrawing through a die f5, or by swaging. Although only a single die isshown in Fig. 1, it will be understood that the drawing operation may berepeated as often as necessary to reduce the assembly to the desiredoutside diameter.

Fig. v3 shows the. mosaic formed as the result of the consolidatingoperation. It will be observed in this gure that the individual Wire I0and their respective coatings I4 form a mosaic and have now assumedasubstantial hexagonal cross section With the coatings taking the formof cell walls", due to the lateral pressure exerted upon them during thedrawing operation.

The consolidated assembly is now subjected to of colloidal the cuttingaction of a saw I6 which cuts oif individual wafers I'I. It will beunderstood that instead of cutting the lassembly in a planeperpendicularto its longitudinal axis, wafers having a surface other than flat mayreadily be formed by machining off discs of the desired shape. Ifdesired, Wafer I'I may be cut with transverse plane surfaces, and thensubjected to modification by a parallel, stepped-displacement of theindividual cellular sections to provide an approximation of a desiredcurvilinear surface. Such a grid structure, after this formingoperation, is shown in section in Fig. 4. In this view, the desiredgeometrical surface contour` is indicated by broken line I9.` If highprecision is necessary,

the surface of the grid structure may readily be machined to this line.

In some cases, it may be desired to provide a grid structure having acurvilinear surface but' in which the individualcellular sections areradially disposed. Such a structurev is illustrated in Fig. 5 of thedrawing. It may bev producedV by first plating or otherwise coating orencasing aA Wafer having transverse plane surfaces with a. matrix ofzinc or other suitable relatively soft material. The coating is madethicker on one face of the Wafer than on the other. The coated wafer isthen pressed into the desired curvilinear orspherical shape, the sidewith the thinner coat.- ing being concave. In this manner, the waferitself is kept under compression in a transverse direction during theshaping operation, so there is no tendency for the individual elementsto sep-f arate as they are displaced. After the coating' material isremoved, the resultant` shaped disc` will comprise a plurality ofindividual sections which have their axes radially disposed with 're-ifspect to the center of curvature of the disc.,

The wafers I'I are now subjected to' preferential or selective etchingin unit |18. The. .purpose rof this operation is to remove or dissolvethe Vmandrel wires I0 while leaving intact their individ-l ual sheathsor coatings I4' to form individual cell walls. If zincwire is used,hydrochloricacid is suitable for performing this operation. vForaluminum Wire, sodium hydroxide is equally suitable.'

The wafers which emerge from u'nit |18 comprise a plurality of 'smalltubular sections or cells which arey movable with respect to each otherso that the same may tend to separate. The walls ofv these individualsections, therefore, are coalesced or otherwise suitably ljoined orbonded together by use of suitable apparatus as indicated at 20. Onewayof accomplishing this is by sintering in a hydrogen furnace. of copper,a sintering temperature between 800 and 900 C. is appropriate. Fornickel, the temperature is about l300 C., and for silver about 500 C. Inaddition tojoining the individual tubular sections or cell wallstogether, the coalescing operation tends to bond the walls of the outergubuar sections to the sheath I3, as indicated in The thickness ofwafers I 'I is preferably so chosen vas to be of the order of theindividual aperture. size of the cells of the finished grid structure.As clearly shown in Fig. 7, each cell or 'tubular section issubstantially square in longitudinal section and has appreciable depthso that the completed grid structure has a maximum structural rigidityreducing microphonic eiects. As also clearly shown in Fig. 7, thethickness of each of the Walls comprising the cells Aof the gridstructure is relatively small compared with the width of each opening.Accordingly', the

If the Vgrids are' overall grid structure has a relatively lowinterception factor when it is interposed in the electron stream of anelectron tube.

Fig. 8 indicates the general appearance of the completed grid structure.If desired, the outer sheath I3 may be silver plated, prior to thecutting operation, in order to provide the proper amount of silver topermit the grid structure to be soldered in place in an electron tube,as for example to the resonator of a klystron. If a rimless gridstructure is desired, outer sheath I3 may be etched away or otherwiseremoved following coalescing.

Several modications in the above-described method of producing cellulargrid structures may be made without departing from the presentinvention. For example, the last two operations illustrated in Fig. 1may be reversed if wires I6 are made of relatively refractory material,such as iron. In this case, the cell walls of the individual tubularsections would be coalesced before their cores were etched or dissolvedaway. Another modification is to accomplish the steps of etching andcoalescing substantially simultaneously. Still another change, which mayin some cases be desirable, is to eifect coalescing prior to the cuttingoperation, this being accomplished, for example, by induction heating ofthe assembly after drawing.

After the cutting operation, it has been found desirable in some casesto remove the feather edge on each wire. This is especially true if thegrid structure is to be bowed or otherwise shaped before the etchingstep, as described above in connection with Figs. 4 and 5. The removalof these feather edges is conveniently accom.- plished by lapping eachwafer with sandpaper or by dropping the wafers into a non-selectivereagent which etches the wires and their coatings more or less equally.

In accordance with the method of the present invention, it has beenfound entirely feasible to make cellular grid structures having webs asthin as 0.0001 inch and to provide such grid structures with at leastone highly precise regular geometrical surface. The utility of suchcellular grid structures in electron tubes, as for example klystrons,will be readily apparent and their use greatly minimizes the productionof secondary electrons due to impact of the primary electrons upon thegrid structure.

Instead of cutting oif wafers, longer portions of desired axial lengthmay be utilized as heat exchangers, chemical filters, or the like. Theclose proximity of adjacent openings of the structure in accordance withthe present invention is especially advantageous in such applications ofthe invention.

The interception factor of a cellular grid structure may, in accordancewith the present invention, be varied without changing the size of wireused merely by omitting the coating on a portion of the wires. In thisway a grid structure will result in which there are a number of openingssubstantially larger than the cross section of the wire afterconsolidation.

While there has been described what is at present considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claim to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

A method of forming curvilinear grid electrodes having a hexagonallyshaped mesh comprising individually coating a plurality of wires of afirst metallic material with a relatively thin coat of second metallicmaterial, assembling said coated wires in a sheath, drawing saidassembly so that said coated wires are deformed into substantiallyhexagonal cross-sections with the abutting surfaces of said coatingscontacting one another and retaining their identities, said coatingsbeing relatively displaceable after said drawing, cutting said drawnassembly transversely to form relatively thin discs, displacing theindividual elements of said discs in a direction substantially at rightangles to the plane of said discs, removing completely said wires offirst material from said discs, and sintering the abutting surfacesamong said coatings and said sheath so that they are coalesced, wherebyan integral, unitary electrode is formed of curved configuration.

SIGURD F. VARIAN. RUSSELL H. VARIAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

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